Techniques developed for the concentration or separation of a solute from its solution are often so expensive and energy-consuming that their application, on a large scale, is limited to the production of substances whose economic, political or social value transcends the criteria of the market place. The desalination of sea water, for example, for the production of potable water or irrigation water for crops, is at present practicable only in a very few arid regions. Different approaches including distillation, electrodialysis based on ion exchange, reverse osmosis using the concentration potential, or vacuum freezing, cannot as yet provide a return that is generally commensurate with the required large investment of capital and energy. Materials problems, such as the difficulty of fabricating durable membranes of the required separating capacity have not yet been satisfactorily solved. In desalination, efficiency of the various technologies are adversely affected by the need to protect machinery against corrosion and scaling, and to absorb or dispose of the process heat before it is added to the environment. Similar considerations impede in many cases the development of large-scale purification, separation and concentration techniques for solutions of diverse composition.
The present invention provides a novel method for effectively concentrating and separating a solute, but requiring only simple apparatus operating under ambient pressure and using relatively low temperatures. The procedure is generally applicable to the separation of a volatile solvent from a solute or to the concentration of a solution with solute and is particularly adaptable for the desalination of sea water.
Specifically, the present invention proceeds by infiltrating solvent-absorbing and gas-entraining matrix material with a solution to be separated and providing a temperature gradient across the matrix material to define hotter and cooler sides. An example of suitable matrix material is stacked sheets of filter paper. As a result of the temperature gradient, which need not be large, a large solute concentration gradient is induced across the matrix material so that solution concentrated with solute can be removed from the hotter side while solution diluted with solvent can be removed from the cooler side. Apparatus provided herein includes a chamber for containing the matrix material and which is formed with appropriate inlet and outlet ports and means for infusing solution into the matrix material.
In its experimental laboratory form, the present process uses apparatus which superficially resembles a Soret cell. In such a cell, a temperature gradient is applied across a solution maintained between a heated upper wall and a cooler lower wall and tends to develop a solute concentration gradient across the solution which is set up by thermal diffusion. The concentration gradient is a function of the solute, its concentration and the temperature and invariably concentrates the solution with solute at the cooler side. The result is that the Soret coefficient is negative, corresponding to a ratio of solute concentration between the heated and cooled surfaces of the Soret cell which is less than unity. Some typical Soret coefficients for different solutions are included in the publication by H. J. V. Tyrell: "Thermal Diffusion Phenomena in the Electrolytes and the Constants Involved," U.S. National Bureau of Standards Circular 524, 119-130 (1953).
The negative nature of the Soret coefficient indicates that the solute component of the solution concentrates at the cooler side. In direct contrast, in accordance with the present invention, by the introduction of a matrix with solvent-absorbing and gas-entraining properties, we have found coefficients which are positive and extraordinarily large. The development of large positive coefficients represents a complete reversal of the concentration ratio in that the solution becomes more concentrated near the heated surface and diluted, i.e., depleted in solute, near the cooler surface. In practice, this enables a drastic separation of solute from solvent in a wide variety of solutions using only low grade temperature levels. For example, sea water, containing NaCl, MgCl, MgSO.sub.4, and a multiplicity of other constituents in small amounts, can be almost totally dissociated into pure drinking water at the cooler surface while a concentrated saline solution can be removed from the heated surface. The separation process can be made continuous by the continuous infusion of fresh solution into the matrix material, and the simultaneous purging of the concentrate and removal of the purified solvent through separate outlets.
The method is effective for separating constituents of a large variety of solutions, a simple limitation being that the solute be less volatile than the solvent. The process lends itself to the separation of multiphase systems including miscible substances in which one species of molecule diffuses more readily through a gaseous phase than another species is able to diffuse through a liquid phase, exemplified by the separation of water (as solute) from ethyl alcohol (as solvent). The method can also be used to concentrate the solute, exemplified by the concentration of maple sugar.
Because of the extremely large positive coefficients obtained with the process provided herein, it can be characterized as a super separation process. The low pressure diffusion flow in the interior of the cell avoids degradation of the matrix, which can be formed of inexpensive cellulose fibers. The maximum temperature of the heated surface must not exceed the boiling point of the solvent, while the lower surface may advantageously be kept at ambient temperature, say 24.degree. C.-32.degree. C. so that there is no need for cooling apparatus; in fact, operation is more efficient when the lower surface is not artificially cooled. The temperature difference between the upper and lower walls of the cell incorporating the matrix material can be in the range of 5.degree. C.-80.degree. C. Such a temperature difference can be easily achieved by utilizing surface and deep ocean water in tropical latitudes or by utilizing low-grade temperature sources to heat the upper surface, such as obtained from solar panels or from the heated effluent of industrial plants or electrical generating plants. Another advantage of the method in accordance with our invention is that clogging of the system is prevented by the continuous infusion of fresh solution, while sumultaneously the concentrated solutes are purged and the diluted solvent is withdrawn from the cell.